This invention relates to data signal correlation detectors, and more particularly, to a MSK data signal correlation detectors.
The abbreviation MSK employed herein means minimum shift keying. A MSK signal may be defined as a signal where the transmitted wave is a phase continuous frequency shift waveform. For each unit interval the instantaneous frequency is a constant being either a high frequency or a low frequency relative to a center frequency. The high frequency is such as to advance one revolution relative to the carrier frequency in four unit intervals. The low frequency is such as to fall behind by one revolution to the carrier frequency in four unit intervals.
Frior art correlators for a MSK data signal requires generation of correlation waveforms. This is accomplished by generating the four amplitude modulated waveforms cos w.sub.o t cos w.sub.m t for 2KT .ltoreq. t &lt; (2K +2)T, sin w.sub.o t cos w.sub.m t for 2DT .ltoreq. t &lt; (2K +2)T, cos w.sub.o t sin w.sub.m t for (2K -1)T .ltoreq. t &lt; (2K +1)T, and sin w.sub.o t sin w.sub.m t for (2K -1)T .ltoreq. t &lt; (2K +1)T over two unit time intervals where the unit time intervals are denoted by T, w.sub.o equals the carrier frequency in radians and w.sub.m equals the modulating frequency in radians. The incoming signal is then correlated against the generated correlation waveforms producing two new correlation coefficients for each unit time interval T.
These devices have the disadvantage that they need analog sum and difference frequency generators, analog gain circuits to produce the sin w.sub.m t or cos w.sub.m t amplitude modulation, analog correlators and have difficult problems in maintaining accurate phase relations.